Objective: To assess the relationship between short sleep duration and obesity-related variables in children involved in the " Québec en Forme " Project. Design: Cross-sectional study. Subjects: A total of 422 children (211 boys and 211 girls) aged between 5 and 10 years from primary schools in the City of Trois-Rivières (Québec) were selected to participate in this study. Measurements: Body weight, height, and waist circumference were measured. The children were classified as normal, underweight, overweight or obèse, according to body mass index per âge. An exhaustive questionnaire was administered by téléphone to the parents of children. Results: The percentage of overweight/obesity was 20.0% in boys and 24.0% in girls. When compared to children reporting 12-13 hours of sleep per day, the adjusted odds ratio for childhood overweight/obesity was 1.42 (95% confidence interval 1.09-1.98) for those with 10.5-11.5 h of sleep and 3.45 (2.61-4.67) for those with 8-10 h of sleep after adjustment for âge, sex, and other risk factors. Parental obesity, low parental educational level, low total family income, long hours of TV watching, playing videogames or computer utilization, absence of breastfeeding, and physical inactivity were also significantly associated with childhood overweight/obesity. In addition, we observed a significant négative association adjusted for âge between sleep duration and body weight (-0.33, p < 0.01), BMI (-0.12, p < 0.01), and waist circumference (-0.24, p < 0.01) in boys. Conclusion: An inverse association was observed between sleep duration and the risk to develop childhood overweight/obesity. Longitudinal research will be required to confirm a potential link of causality between thèse variables.